Revert "Remove useless assignation"

Revert "Remove useless assignation"

This reverts commit 338355426e19d4cb0d92a632a44726c7f249fdcd.
It segfaults as p_next is now uninitialized.

==25869== Conditional jump or move depends on uninitialised value(s)
==25869==    at 0x5078332: GetNextItem (tree.c:441)
==25869==    by 0x5078D7A: playlist_GetNextLeaf (tree.c:362)
==25869==    by 0x5071CFE: ResetCurrentlyPlaying (thread.c:197)
==25869==    by 0x5072952: Thread (thread.c:596)
==25869==    by 0x577AFC6: start_thread (in /lib/libpthread-2.7.so)
==25869==    by 0x5EE65AC: clone (in /lib/libc-2.7.so)